The present invention disclosed herein relates to a wavefront sensor, and more particularly, to an operating method of a first derivative measuring apparatus.
Various interferometers including a Michelson interferometer and a Mach-Zehnder interferometer compare a reference wavefront and a measurement wavefront and correct an error included in the measurement wavefront. On the contrary, a lateral shearing interferometer detects a slope value of a measurement wavefront without a separate reference wavefront to measure an error. In detail, the lateral shearing interferometer separates a measurement wavefront of a target into two identical wavefronts, and allows one of the two wavefronts to be sheared in a lateral direction. The lateral shearing interferometer mixes the sheared wavefront and an original wavefront to cause interference, and measures the interference pattern. The interference pattern measured at this time includes slope information about the measurement wavefront. The lateral shearing interferometer obtains the slope information of the measurement wavefront, integrates the obtained information over the entire domain, and obtains an error of the measurement wavefront. First derivative measuring devices may operate by above-described scheme. The first derivative measuring devices may include various devices such as a deflectometry, a lateral shearing interferometer, and a Shack-Hartmann sensor.
These first derivative measuring devices obtain slope information of a measurement wavefront by rotating targets or devices through 90° in a direction perpendicular to a reference direction. In this procedure, when the rotation angle is not precisely 90°, there may be an error between the measured wavefront and an actual wavefront.